You're trying to extract the data from the image file?
imread will work find for this purpose. It will return an mxnx3 variable where each page represents the red, green, and blue values for each pixel respectively.
Or do you already have the data in MATLAB and you only want to take a small chunk of it? In that case, it would help to know the shape of the data. But I'll assume for the moment that you've got two vectors, LAT, and LON that represent the ranges of latitude and longitude and a 2D variable BATH. Let's assume LAT is a column vector, and LON is a row vector. BATH should be [length(LAT) length(LON)] in size.
LAT_index = LAT >= 55 & LAT <= 60;
LON_index = LON >= -8 & LON <= -4;
LAT_subset = LAT(LAT_index);
LON_subset = LON(LON_index);
BATH_subset = BATH(LAT_index,LON_index);